The parenteral, in particular the intravenous, administration of water-insoluble or poorly water-soluble substances such as drugs or other biological materials often presents a problem to the formulator. Since the diameter of the smallest blood capillaries is only a few microns the intravenous application of larger particles would lead to capillary blockage. Solid drug substances are, however, commonly disintegrated by milling and grinding, thereby generating particles from a few millimeters down to the micrometer size range which are too large to be injected directly as an aqueous suspension. As a consequence, intravenous administration systems containing suspended particles of water-insoluble drugs are not commercially available due to the risk of embolism. A further decrease in particle size is expensive, ineffective or even impossible by conventional techniques. Additionally, the reduction of solids to submicron-sized powders brings about heavy difficulties in handling these dry products such as an increased risk of dust explosions and cross-contamination problems in a factory environment. Moreover, such systems present a health risk for persons exposed to the possible inhalation and absorption of potent bioactive materials. Up to now the only possibilities to administer poorly water-soluble substances by the intravenous route are the use of co-solvents or the development of carrier systems which incorporate such substances in vehicles with hydrophilic surfaces.
Basic requirements of an ideal drug carrier system imply biodegradability, non-toxicity and non-immunogenicity. Moreover, the carrier should be suitable for the intended route of administration, e g with regard to particle size. Often a controlled release of the incorporated bioactive material is desired, for example when constant serum levels should be maintained over a long period of time or when the drug exhibits only a low therapeutic index.
Furthermore, carrier systems can be employed to prolong the half-life of certain substances which are unstable due to rapid enzymatic or hydrolytic degradation in biological milieu. On the other hand the incorporation of drug in the carrier material also presents an opportunity to protect the host from the drug in case of non-selective toxic substances such as antitumour agents.
In many cases drug carrier systems are developed with the object to deliver drugs to site-specific targets under circumvention of uptake by the reticuloendothelial system (RES). The rationale for such a drug targeting is an enhancement of the drug's therapeutic efficacy by an increase of the drug concentration at the target site with a simultaneous decrease at non-target sites, thereby rendering possible a reduction of the administered dose. Thus, the toxicity of drugs, e g anticancer agents, can be diminished, leading to a decrease of side effects.
The prerequisite of a successful site-specific delivery implies a certain selectivity of the carrier system for the target tissue as well as the accessibility of the desired target site. Targeting by the intravenous route of application is generally connected to an avoidance or at least a reduction of carrier uptake by the RES except for the cases where a direct targeting to cells of the RES is desired. Clearance of colloidal particles by the RES has been described to depend on particle size as well as on particle surface characteristics such as surface charge and surface hydrophobicity. In general, small particles are cleared less rapidly from the blood stream than large particles whereas charged particles are taken up more rapidly than hydrophilic non-charged particles. Due to these facts approaches to drug targeting are the modification of surface characteristics and the reduction of particle size.
Moreover, a small particle size is also required for the targeting of drugs to extravascular sites since extravasation is only feasible through a receptor-mediated uptake by phagocytosis/pinocytosis or where the endothelial wall is fenestrated. These fenestrations can be found for example in the sinusoids of liver, spleen and bone-marrow and show diameters of up to approximately 150 nm.
From the manufacturing point of view the ideal drug carrier system should be preparable without complications by easy-to-handle techniques in a reproducible manner and possibly at low production costs. The formulation should exhibit sufficient stability during preparation as well as on storage.
In recent years several colloidal systems have received special interest for their potential application as drug carriers, among them being liposomes, lipid emulsions, microspheres and nanoparticles. However, all of the systems mentioned possess a certain number of draw-backs which so far have prevented the break-through of any such system as a wide-spread, commercially exploited drug carrier.
Drug carrier systems in the micrometer size range are represented by microspheres consisting of a solid polymer matrix, and microcapsules in which a liquid or a solid phase is surrounded and encapsulated by a polymer film. Nanoparticles consist, like microspheres, of a solid polymer matrix. Their mean particle size, however, lies in the nanometer range. Both micro- and nanoparticles are generally prepared either by emulsion polymerization or by solvent evaporation techniques. Due to these production methods micro- and nanoparticles bear the risk of residual contaminations from the production process like organic solvents such as chlorinated hydrocarbons, as well as toxic monomers, surfactants and cross-linking agents, which may lead to toxicological problems. Moreover, some polymeric materials such as polylactic acid and polylactic-glycolic acid degrade very slowly in vivo so that multiple administration could lead to polymer accumulation associated with adverse side effects. Other polymers such as polyalkylcyanoacrylates release toxic formaldehyde on degradation in the body.
Drug carrier systems for parenteral administration based on lipids are liposomes and submicron lipid emulsions. Although such systems consist of physiological components only, thus reducing toxicological problems there is a number of disadvantages associated with these lipid carriers.
Liposomes are spherical colloidal structures in which an internal aqueous phase is surrounded by one or more phospholipid bilayers. The potential use of liposomes as drug delivery systems has been disclosed inter alia in the U.S. Pat. Nos. 3,993,754 (issued Nov. 23, 1976 to Rahmann and Cerny), 4,235,871 (issued Nov. 25, 1980 to Papahadjopoulos and Szoka) and 4,356,167 (issued Oct. 26, 1982 to L. Kelly). The major drawbacks of conventional liposomes are their instability on storage, the low reproducibility of manufacture, the low entrapment efficiency and the leakage of drugs.
According to the IUPAC definition, in an emulsion liquids or liquid crystals are dispersed in a liquid. Lipid emulsions for parenteral administration consist inter alia of liquid oil droplets, predominantly in the sub-micron size range, dispersed in an aqueous phase and stabilized by an interfacial film of emulsifiers. Typical formulations are disclosed in the Jap. Pat. No. 55,476/79 issued May 7, 1979 to Okamota, Tsuda and Yokoyama. The preparation of a drug containing lipid emulsion is described in WO 91/02517 issued Mar. 7, 1991 to Davis and Washington. The susceptibility of these lipid emulsions towards the incorporation of drugs is relatively high due to the mobility of drug molecules within the internal oil phase since diffusing molecules can easily protrude into the emulsifier film causing instabilities which lead to coalescence. Furthermore, release of incorporated drugs from lipid emulsions is relatively fast so that the possibilities for a sustained drug release are limited.
Fountain et al (U.S. Pat. No. 4,610,868 issued Sep. 9, 1986) developed lipid mhatrix carriers which are described as globular structures of a hydrophobic compound and an amphiphatic compound with diameters from about 500 nm to about 100.000 nm. The hydrophobic compound can be liquid or solid. The preparation techniques, however, employ organic solvents and are thus associated with the problem of complete solvent removal.
So-called lipospheres disclosed by Domb et al (U.S. Pat. Appl. No. 435,546 lodged Nov. 13, 1989 and now abandoned; Int. Appl. No. PCT/US90/06519 filed Nov. 8, 1990) are described as suspensions of solid, water-insoluble microspheres made of a solid hydrophobic core surrounded by a phospholipid layer. Lipospheres are claimed to provide for the sustained release of entrapped substances controlled by the phospholipid layer. They can be prepared by a melt or by a solvent technique, the latter creating toxicological problems if the solvent is not completely removed.
A slow release composition of fat or wax and a biologically active protein, peptide or polypeptide suitable for parenteral administration to animals is disclosed in U.S. Pat. Appl. No. 895,608 lodged Aug. 11, 1986, and now abandoned, to Staber, Fishbein and Cady (EP-A-0 257 368). The systems are prepared by spray drying and consist of spherical particles in the micrometer size range up to 1,000 microns so that intravenous administration is not possible.
Problems with the formulation of water-insoluble or poorly water-soluble substances are not restricted to the parenteral route of administration. Thus, the peroral bioavailability of drugs is related to their solubility in the gastrointestinal tract (GIT), and it is generally found that poorly water-soluble drugs exhibit a low bioavailability. Moreover, the dissolution of drugs in the GIT is influenced by their wettability. Substances with apolar surfaces are scarcely wetted in media so that their dissolution rate is very slow.
In an attempt to improve the intestinal absorption of lipophilic drugs, Eldem et al (Pharm. Res. 8, 1991, 47-54) prepared lipid micropellets by spray-drying and spray-congealing processes. The micropellets are described as spherical particles with smooth surfaces. The lipids are, however, present in unstable polymorphic forms, and polymorphic phase transitions occur during storage so that the product properties are constantly changing (T. Eldem et al, Pharm. Res. 8, 1991, 178 -184). Thus, constant product qualities cannot be assured.
Lipid nanopellets for peroral administration of poorly bioavailable drugs are disclosed in EP 0 167 825 of Aug. 8, 1990 to P. Speiser. The nanopellets represent drug-loaded fat particles solid at room temperature and small enough to be persorbed. Persorption is the transport of intact particles through the intestinal mucosa into the lymph and blood compartment. The lipid nanopellets are prepared by emulsifying molten lipids in an aqueous phase by high-speed stirring. After cooling to room temperature the pellets are dispersed by sonication.